1. Field of the Invention
The present invention relates to an image processing apparatus and method which, when a printing apparatus prints on a print medium, reduces density unevenness resulting from print characteristic variations among a plurality of printing elements in a print head, from undulations of print head scans or from print medium conveying operations.
2. Description of the Related Art
As an example of printing system using a print head having a plurality of printing elements, an ink jet printing system has been known which ejects ink from the individual printing elements to form dots on the print medium.
A serial type ink jet printing apparatus progressively forms an image by repetitively alternating a printing scan, which moves a print head ejecting ink at a predetermined frequency over a print medium at a speed that matches the frequency, and a print medium conveying operation, which conveys the print medium in a direction that crosses the direction of the printing scan.
In a print head having a plurality of printing elements arrayed therein, some variations unavoidably occur among printing elements during a manufacturing process. Such variations show up as ejection characteristic variations, such as ink ejection volume variations and ink ejection direction variations, rendering the shapes of dots formed on a print medium unequal, thereby inducing density (or grayscale) unevenness and stripes on the printed image.
In an attempt to address this problem, the serial type ink jet printing apparatus adopts a characteristic multipass printing. The multipass printing prints an image by dividing the pixels that the print head can print in one printing scan into two or more groups of image data and printing them progressively in a plurality of printing scans, with the paper conveying operation interposed in between. This printing method, if there are ejection characteristic variations among individual printing elements, can prevent dots printed by one printing element from continuing in the main scan direction and thereby disperse the influences of ejection characteristics of individual printing elements over a wide area, producing a uniform, smooth image. The desirable effect of such a multipass printing increases with a number of passes, that is the number of printing elements set to print one main scan line. It is noted, however, that an increased number of passes results in a reduced print speed. So, a serial type printing apparatus often provides a plurality of print modes with different numbers of passes, such as one giving priority to an image quality and one giving priority to a print speed. Further, in recent years, a printing apparatus has come to be marketed which has a plurality of printing element arrays (or print heads) arranged side by side in the main scan direction to eject the same inks in order to realize a relatively high-speed image output even while performing the multipass printing.
When such a multipass printing is performed or a plurality of print heads are used, print data needs to be distributed to individual printing scans or individual print heads. Such a print data distribution conventionally has often been done by using a mask pattern that has a predetermined pattern of pixels (1) that permit a dot to be printed therein and pixels (0) that do not.
FIG. 2 is a schematic diagram showing an example of mask pattern that may be used in a 2-pass printing. Areas painted black represent pixels (1) that permit a dot to be printed therein and blank areas represent pixels (0) that do not permit a dot to be printed therein. Denoted 201 is a mask pattern used in a first pass of printing scan and 202 a mask pattern used in a second pass of printing scan. The first-pass pattern 201 and the second-pass pattern 202 are in a complementary relationship.
Binary image data to be printed is ANDed with the above-described mask patterns and the result of the AND operation constitutes binary data that will be actually printed by individual printing elements in their associated printing scans. However, since the distribution of pixels to be printed differs from one piece of binary image data to another, it is difficult to distribute the print data evenly among a plurality of printing scans or a plurality of printing element arrays at all times. As the percentage or frequency of a particular printing scan or a particular printing element array printing dots increases, the ejection characteristic of that printing scan or printing element array shows in a printed image, diminishing the intended advantage of the multipass printing. So, when performing the multipass printing, one important issue that needs to be resolved has been how the print data can be distributed among a plurality of printing scans or printing element arrays evenly and uniformly.
For example, Japanese Patent Laid-Open No. 7-52390 discloses a method for generating a mask pattern that has print-permitted pixels and print-not-permitted pixels randomly arrayed. The use of such a random mask pattern can be expected to distribute the print data almost evenly among a plurality of printing scans or a plurality of printing element arrays, by whatever quantization method the image data is binarized.
Further, Japanese Patent Laid-Open No. 6-191041 discloses a method which does not use a fixed mask pattern, such as shown in FIG. 2, but distributes print data among printing scans so that pixels to be printed adjoining continuously in the main scan direction or subscan direction are printed by as many different printing scans as possible.
FIG. 3 shows an example array of pixels to be printed in binary image data and a result of distributing the pixels to two printing scans according to the method described in Japanese Patent Laid-Open No. 6-191041. Having dots that run continuously in the main scan direction and subscan direction printed in different printing scans as described above can effectively alleviate not just image impairments caused by ink ejection characteristic variations among printing elements but also problems such as ink overflows.
As still higher image quality is being demanded of printed images in recent years although the above-described multipass printing has already been employed, grayscale variations and density unevenness caused by registration shifts or deviations among printing scans or among printing element arrays have come to be viewed as new problems. The registration shifts or deviations among printing scans or among printing element arrays are caused by variations in distance between a print medium and an ejection opening face of the print head (paper-print head distance) or by variations in distance that the print medium is conveyed, and appear as deviations between different planes that are printed by individual printing scans or printing element arrays.
Referring to FIG. 3, consider an example case in which a plane of dots (circle) printed in a preceding printing scan and a plane of dots (double circle) printed in a subsequent printing scan are shifted by one pixel in the main scan direction or subscan direction. In this case the dots (circle) printed in the preceding printing scan and the dots (double circle) printed in the subsequent printing scan completely overlap, exposing blank areas, lowering the grayscale level of the printed image. Variations in distance between adjoining dots and in their overlapping portions, though not as large as one pixel, have great effects on the dot coverage over blank areas and therefore the image grayscale level. That is, if the shift between different planes changes due to variations in the distance between the print medium and the ejection opening face (paper-print head distance) and variations in print medium conveyed distance, a uniform grayscale level of an image also changes, resulting in grayscale variations or density unevenness emerging in the printed image.
Therefore, as higher and higher quality is being demanded of printed images in recent years, there is a growing need for a print data distribution method used in multipass printing that can deal with possible registration shifts between different planes caused by variations in many printing conditions. An ability to tolerate grayscale variations and density unevenness in printed images resulting from registration shifts between different planes, no matter what printing condition variations have caused the registration shifts, is referred to as a “robustness” in this patent application.
Japanese Patent Laid-Open No. 2000-103088 discloses a method of distributing print data to enhance the robustness. This patent document focuses its attention on the fact that image grayscale variations resulting from variations in many printing conditions are caused by the distributed pieces of binary print data being in a complete complementary relationship with each other. The patent document discloses a technique which, in an attempt to reduce such a complementary relationship to prevent large grayscale variations from occurring when there are shifts among a plurality of planes, involves distributing the image data corresponding to individual pixels in the form of multivalued data before being binarized and then individually binarizing the distributed multivalued data.
FIG. 4 is a block diagram showing an example of control configuration that realizes the data distribution as disclosed in Japanese Patent Laid-Open No. 2000-103088. The patent document describes as an example a printing apparatus that distributes print data to two print heads. Multivalued image data from a host computer 2001, after being subjected to a variety of image processing, is distributed to a first data conversion unit 2008 and a second data conversion unit 2009 according to a predetermined distribution ratio by a multivalued SMS unit 2007. Each of the data conversion units performs a conversion operation on the data according to the specified distribution coefficient, with the converted multivalued data transferred to a first binarization unit 2010 and a second binarization unit 2011. The first binarization unit 2010 and the second binarization unit 2011 each perform the binarization based on an error diffusion method using an error matrix and a threshold. The binarized image data are stored in a first band memory 2012 and a second band memory 2013. Then, in a predetermined printing scan the associated print heads eject ink according to the binary data stored in the respective band memories.
FIG. 5 shows an array of dots printed on a print medium according to Japanese Patent Laid-Open No. 2000-103088. In the figure, black dots 21 are those printed by a first print head, white dots 22 are those printed by a second print head and hatched dots 23 are those printed overlappingly by the first print head and the second print head. Unlike FIG. 3 that has a complete complementary relationship between the dots printed by the first print head and the dots printed by the second print head, this example has no such complementary relationship, so in some areas two dots overlap and in other areas or blank areas not a single dot is formed.
Here, let us consider a case where a plane printed by the first print head and a plane printed by the second print head are shifted by a distance of one pixel either in the main scan direction or in the subscan direction. In that case, while areas printed overlappingly by the first print head and the second print head increase, there also occur areas where the two dots already printed overlappingly come apart. So, when a certain expanse of area is considered, the dot coverage over a white print medium has not changed much, nor has a change occurred in an image grayscale level. That is, the method proposed by Japanese Patent Laid-Open No. 2000-103088 can suppress grayscale level variations and occurrence of density unevenness even if the distance between the print medium and the print head ejection face (paper-print head distance) changes or if the print medium conveyed distance changes.
Further, Japanese Patent Laid-Open No. 2006-231736 discloses a technique which, while distributing the image data in a multivalued state among a plurality of printing scans or printing element arrays as Japanese Patent Laid-Open No. 2000-103088 does, changes the distribution ratio of the image data according to the position of pixel of interest. In this patent document, the effects of suppressing banding and color variations that would occur during a multipass printing are described to be able to be produced by changing the distribution ratio according to the pixel positions in the main scan direction, linearly, cyclically, sinusoidally or based on a combination of high and low frequency waves.
However, studies conducted by the inventors of this invention have found that, even if it employs the methods of Japanese Patent Laid-Open No. 2000-103088 and 2006-231736, the printing system that distributes the image data among a plurality of planes before printing leaves image impairments in an output image. The image impairments produced will be described in detail as follows. In the methods of Japanese Patent Laid-Open No. 2000-103088 and 2006-231736, the binarization operations in individual planes are performed independently, with no correlation provided between them. More specifically, for example, the decision of whether a dot should be or should not be printed in a pixel of interest on a certain plane does not consider or use information on a pixel that lies at the same position in other planes.
In such circumstances, if deviations occur between different planes after the binarization operations have been performed, the low frequency component of dots arrangement on individual planes may become emphasized, showing its characteristic pattern (texture), which in turn may be recognized as graininess, or unwanted image impairments. That is, the methods of Japanese Patent Laid-Open Nos. 2000-103088 and 2006-231736, while they can suppress grayscale variations caused by shifts between different planes, cannot eliminate the image impairment that is visually recognized as graininess, failing to provide enough robustness.